Generally, a powershift transmission includes a number of gear elements coupling the input and output shafts, and a related number of clutches which are selectively engageable to activate gear elements for establishing a desired speed ratio between the input and output shafts. The clutch may be of the band or disk type.
For example, the input shaft may be connected to the engine through a fluid coupling, such as a torque converter, and the output shaft is connected directly to the vehicle drive. Shifting from one gear ratio to another involves releasing or disengaging the off-going clutches associated with the current gear ratio and applying or engaging the on-coming clutches associated with the desired gear ratio.
To improve the shifting of the transmission, several transmission manufactures have utilized electronic control technology. Electronic control technology employs a direct interface between an electronic control unit and the clutches via a plurality of solenoid valves. The solenoid valves are modulated to control both the on-coming and off-going clutch pressures in response to command signals from the electronic control unit.
To provide for precise timing of the shifting, it is desirous to determine the fill time of an on-coming clutch. The fill time is the time required to fill an on-coming clutch cavity with fluid. During the fill period, the clutch piston strokes and the clutch plates compress. However, the clutch cannot transmit significant torque until the compression is complete. Thus, for reliable clutch control during shifting, it is desirous to accurately determine the length of the fill period so as to know when the clutch is ready to transmit significant torque.